Profit from sale of flat

Capital gain tax will be payable on profit from sale of flat.

It depends on what sort of transaction you have had with the buyer.
 

Actually I have invested that profit in transaction, to Purchase a New flat  

U/s 54 Any long term capital gains arising on the transfer of a residential flat to an individual, will be exempt from tax if the assessee has within a period of 1 year before or 2 years after the date of such transfer purchased, or within a period of 3 years constructed, a residential house.